Isopropylidene bis(phthalic acid), also known as 2,2-bis-(3,4-dicarboxyphenyl)propane (IBPA), is a useful chemical intermediate. For example, this tetracarboxylic acid can be dehydrated to produce isopropylidene bis(phthalic anhydride) (IPAN) from which polymers with enhanced blending properties in polyether ketone formulations can be prepared. Alternatively, the dianhydride can be converted to an ester which can then be used in the manufacture of polyimide resin having a reduced dielectric constant and useful in electronic composites.
Oxidation processes for producing IBPA from dixylylpropane (DXP), also known as isopropylidene bis(xylene) and as 2,2-bis-(3,4-dimethylphenyl)propane, have previously been described in, for example, Gresham et al. U.S. Pat. No. 2,712,543 where nitric acid and also potassium permanganate oxidation of DXP to IBPA are taught. However, both these oxidation processes consume relatively large amounts of inorganic reagents, involve elaborate procedures, and produce IBPA yields of questionable economic value, as well as a relatively large number of undesired by-products.
A possibly more promising oxidative route for preparing IBPA from DXP would be to utilize an alkyl aromatic oxidation process such as the so-called "Mid-Century Oxidation Process" generally described by Towle et al., "Make Most Aromatic Acids Using Mid-Century Oxidation Processes", in "Petrochemical Developments", November, 1964, Vol. 43, No. 11, pp. 149-152. See also Baldwin U.S. Pat. No. 3,064,044, Marsh et al. U.S. Pat. No. 4,081,464, and Brown et al. U.S. Pat. No. 4,587,355. However, liquid phase, elevated temperature oxidation of DXP using a cobalt/manganese/bromine catalyst as in the Mid-Century oxidation process is not very efficient for the manufacture of IBPA. Relatively low yields of IBPA are obtained, and unacceptably high amounts of an unwanted by-product, trimellitic acid, are produced.
A new and improved oxidation process for converting DXP to IBPA would be economically advantageous. The present invention provides such a process.